1. Field of the Invention
The present invention relates to technologies for detecting radiant rays, for example, infrared rays emitted from a human body or the like, and in particular, to a technology suitable for detecting a still living body with a simple structure and high precision while preventing erroneous detection due to thermal interference.
2. Description of the Related Art
To detect a living body according to variations in heat emitted from the living body such as a human body, a known technology uses a sensor element such as a pyroelectric element, a thermopile, a bolometer, and a thermistor.
A pyroelectric element (hereinafter this infrared-ray detecting element is referred to as an AC infrared-ray detecting element) is a non-contact temperature sensor that detects variations in the amount of heat generated when a human body enters. On the other hand, an infrared-ray detecting element (hereinafter the infrared-ray detecting element is referred to as a DC infrared-ray detecting element) such as a thermopile and a thermistor is a non-contact temperature sensor that detects the amount of heat emitted from a human body.
Accordingly, the AC infrared-ray detecting element does not output a signal and cannot detect a human body if the human body is standing still and variations in the amount of heat are not detected. On the other hand, the DC infrared-ray detecting element outputs a signal according to the amount of heat emitted from a human body and thus can detect the human body even if the human body is standing still.
A technology of an infrared-ray human body detecting sensor using such a DC infrared-ray detecting element is disclosed in Patent Document 1. That is, Patent Document 1 discloses the technology that uses a pair of a positive-polarity infrared-ray sensor and a negative-polarity infrared-ray sensor to prevent erroneous detection due to thermal interference. Thus, this technology prevents erroneous detection in which thermal interference, such as temperature variations in the sensor itself and uniform temperature variations in a detection range, is detected as being caused by a human body.
Specifically, the multi-element human body detecting sensor disclosed in Patent Document 1 has at least a pair of the positive-polarity and negative-polarity infrared-ray sensors. By calculating a total sum of the output voltages of the infrared-ray sensors, the human-body detecting sensor can prevent the erroneous detection due to the thermal interference.
However, the human body detecting sensor disclosed in Patent Document 1, which uses a pyroelectric element, a thermopile, or a thermistor as the positive-polarity and negative-polarity infrared-ray sensor, has the following problems. A first problem accompanying the use of the pyroelectric element is that a chopper is required to detect a still living body. A second problem accompanying the use of the thermopile is that a heat insulating property is poor and sensitivity becomes low because a hot junction and a cold junction are connected to each other by the material of the thermopile. A third problem accompanying the use of the bolometer or the thermistor is that it is hard to equalize the absolute values of positive and negative output temperature coefficients because the positive-polarity and negative-polarity sensors are required to be made of the same material.
Further, for the purpose of detecting a temperature with high precision using a sensor taking the shape of a monolithic integrated circuit, Patent Document 2 discloses a work-function-difference-based temperature sensor that is composed of plural MOSFETs and uses the temperature characteristic of a work function difference between the gate electrodes of the respective MOSFETs; and a work-function-difference-based infrared-ray sensor and a human-body detecting sensor to which the work-function-difference-based temperature sensor is applied. Note that a work function is energy necessary for extracting one free electron in a solid object from a substance at 0 K, which is expressed in terms of a potential difference (V) and refers to the absolute value of the energy of a Fermi level measured when the energy level of electrons in vacuum is zero.
However, the technology disclosed in Patent Document 2 cannot prevent the erroneous detection in which the thermal interference, due to temperature variations in the sensor itself and uniform temperature variations in a detection range, is detected as being caused by a human body or the like.
According to the conventional technologies, it is not possible to easily manufacture a sensor that detects a still living body with a simple structure and high precision while preventing the erroneous detection due to the thermal interference.
Patent Document 1: JP-A-11-108754
Patent Document 2: JP-A-2007-101213